Polar ft4 not working

Any advice. I can't get mine to work. I have changed all battieries and even bought a new strap. I just got the new strap and yesterday I had a bit of a hard time getting it to get come on (not show zero has my heart rate) but then it seemed to work for my body combat class.. today.. I did my body pump ab dvd for 15 minutes and I was shocked it gave me almost 200 calories for that short workout. So. I checked the aveage and max heart rate. Average 190 and max over 200. So I know it's not working right.

What would be my next step to do? I am close to my goal weight and really would like to know my numbers are correct when I move into maintaining my loss.